Cellular base stations typically include, among other things, a radio, a baseband unit, and one or more antennas. The radio receives digital information and control signals from the baseband unit and modulates this information into a radio frequency (“RF”) signal that is transmitted through the antennas. The radio also receives RF signals from the antenna and demodulates these signals and supplies them to the baseband unit. The baseband unit processes demodulated signals received from the radio into a format suitable for transmission over a backhaul communications system. The baseband unit also processes signals received from the backhaul communications system and supplies the processed signals to the radio. A power supply may also be provided that generates suitable direct current (“DC”) power signals for powering the baseband unit and the radio. For example, the radio is often powered by a (nominal) 48 Volt DC power supply in cellular systems that are currently in use today. A battery backup is also typically provided to maintain service for a limited period of time during power outages.
In order to increase coverage and signal quality, the antennas in many cellular base stations are located at the top of an antenna tower, which may be, for example, about fifty to two hundred feet tall. Antennas are also routinely mounted on other elevated structures such as, for example, buildings, utility poles and the like. Until fairly recently, the power supply, baseband unit and radio were all located in an equipment enclosure at the bottom of the antenna tower or other elevated structure to provide easy access for maintenance, repair and/or later upgrades to the equipment. Coaxial cable(s) were routed from the equipment enclosure to the top of the antenna tower and were used to carry RF signals between the radios and the antennas.
FIG. 1 is a schematic diagram that illustrates a conventional cellular base station 10. As shown in FIG. 1, the depicted cellular base station 10 includes an equipment enclosure 20 and an antenna tower 30. The equipment enclosure 20 is typically located at the base of the antenna tower 30, as shown in FIG. 1. A baseband unit 22, a radio 24 and a power supply 26 are located within the equipment enclosure 20. The baseband unit 22 may be in communication with a backhaul communications system 44. A plurality of antennas 32 (e.g., three sectorized antennas 32-1, 32-2, 32-3) are located at the top of the antenna tower 30. Three coaxial cables 34 (which are bundled together in FIG. 1 to appear as a single cable) connect the radio 24 to the antennas 32. The antennas 32 are passive (unpowered) devices and hence none of the equipment at the top of the tower 30 requires electrical power. While the cellular base station 10 of FIG. 1 (and various other cellular base stations shown in subsequent figures) is shown as a having a single baseband unit 22 and radio 24 to simplify the drawings and description, it will be appreciated that cellular base stations routinely have multiple baseband units 22 and radios 24 (and additional antennas 32), with three or more baseband units 22 and radios 24 being common in state-of-the-art systems. There is a practical limit, however, to the number of devices that can be housed in the enclosure 20.
In recent years, a shift has occurred and the radio 24 is now more typically located at the top of the tower 30 in new or upgraded cellular installations. Radios that are located at the top of the tower 30 are typically referred to as remote radio units or remote radio heads (“RRH”) 24′. Using remote radio heads 24′ may significantly improve the quality of the cellular data signals that are transmitted and received by the cellular base station, as the use of remote radio heads 24′ may reduce signal transmission losses and noise that can occur in the transmission line between the radios 24 and the antennas 32. In particular, as the coaxial cables 34 that connect radios 24 that are located at the base of an antenna tower 30 to antennas 32 that are mounted near the top of the antenna tower 30 may have lengths of 100-200 feet or more, the signal loss that occurs in transmitting signals at cellular frequencies (e.g., 1.8 GHz, 3.0 GHz, etc.) over these coaxial cables 34 may be significant, as at these frequencies the coaxial cables 34 tend to radiate RF signal energy. Because of this loss in signal power, the signal-to-noise ratio of the RF signals may be degraded in systems that locate the radio 24 at the bottom of the antenna tower 30 as compared to cellular base stations having remote radio heads 24′ that are located at the top of the tower 30 next to the antennas 32 (note that signal losses in the cabling connection between the baseband unit 22 at the bottom of the tower 30 and the remote radio head 24′ at the top of the tower 30 may be much smaller, as these signals are transmitted at baseband or intermediate frequencies as opposed to RF frequencies, and because these signals may be transmitted up the antenna tower 30 over fiber optic cables, which may exhibit lower losses).
FIG. 2 is a schematic diagram that illustrates a cellular base station 10′ according to this newer architecture. As shown in FIG. 2, the baseband unit 22 and the power supply 26 may still be located at the bottom of the tower 30 in the equipment enclosure 20. The radio 24, in the form of a remote radio head 24′, is located at the top of the tower 30 immediately adjacent to the antennas 32. While the use of tower-mounted remote radio heads 24′ may improve signal quality, it also requires that DC power be delivered to the top of the tower 30 to power the remote radio head 24′. As shown in FIG. 2, typically a fiber optic cable 38 connects the baseband unit 22 to the remote radio head 24′ (as fiber optic links may provide greater bandwidth and lower loss transmissions), and a separate or combined (“composite”) power cable 36 is provided for delivering the DC power signal to the remote radio head 24′. The separate power cable 36 is typically bundled with the fiber optic cable 38 so that they may be routed up the tower 30 together. In other cases (not shown), a hybrid fiber optic/power trunk cable 40 may be run up the tower 30. Such trunk cables 40 typically have junction enclosures on either end thereof, and a first set of data and power jumper cables are used to connect the junction enclosure on the ground end of the trunk cable 40 to the baseband unit(s) 22 and power supply 26, and a second set of data and power (or combined data/power) jumper cables are used to connect the junction enclosure at the top of the tower 30 to the remote radio heads 24′.
Another change that has occurred in the cellular industry is a rapid increase in the number of subscribers, as well as a dramatic increase in the amount of voice and data traffic transmitted and received by a typical subscriber. In response to this change, the number of remote radio heads 24′ and antennas 32 that are being mounted on a typical antenna tower 30 has also increased, with twelve remote radio heads 24′ and twelve or more antennas 32 being a common configuration today.
Moreover, because of the desire to provide network coverage over a large geographic area, cellular base stations are often located in relatively remote locations. In order to ensure continuity of service in the event of a power outage, cellular base stations may be provided with a backup battery 43 that can supply DC power in the event an external power supply, such as an AC line voltage, is interrupted. As illustrated in FIG. 2, the backup battery 43 may be physically located in the enclosure 20 and may be connected to the power supply 26. When the power supply 26 detects a loss of AC line power, the power supply may supply the DC signal from the battery 43 to the RRH 24′ over the cable 40.
In addition, other types of radio equipment, such as terrestrial microwave (TMW) radios 25, may be co-located on a cellular antenna tower 30 along with the remote radio heads 24′. This equipment also receives power from the power supply 26. Remote radio heads 24′, terrestrial microwave transceivers 25, and other powered equipment that may be located on a tower 30 are generally referred to herein as “tower top equipment” or TTE.